Numbers of mechanical positioning systems requiring the highest available accuracy utilize air bearings, to minimize friction and backlash effects, together with laser interferometers to determine position as proposed for example by Hinds in U.S. Pat. No. 4,654,571. However such interferometer systems are of necessity costly and complex and exhibit further difficulties if it is desired to position several such elements independently in a common working area. This latter problems arises due to the inevitable mutual blocking of the multiple independent measuring light beams for some relative positions of the moving elements. The need for such multiple independent elements can arise, for example, in certain types of high precision automated fabrication and assembly applications.
An attractive alternative approach would be to arrange that each moving element be capable of determining its own position directly, in two dimensions, relative to a patterned working surface over which it moved. Suitable patterns arise automatically if two-dimensional actuators of the type initially proposed by Sawyer in U.S. Pat. No. Re 27,436 are employed. However, no prior method exists for measuring position accurately, rapidly and at low cost over large working regions for such actuators or, indeed for any multiple independent moving elements.
In other contexts, capacitive transducers are known to be advantageous. The use of capacitive transducers for distance measurement is an established technique, and is capable of providing exceedingly high resolution as discussed by R. V. Jones and J. C. S. Richards, "The Design and Application of Capacitance Micrometers", Journal of Physics E, Science Instruments, Vol. 6 Series 2, 1973, pp. 589-600, and A. M. Thompson, "The Precise Measurement of Small Capacitances", IRE Transactions on Instrumentation, Vol. 1-7, 1958, pp. 245-253. This high sensitivity is inherent and derives in the last resort from the fact that capacitors themselves generate no noise. Such capacitive methods are well known and have even been extended to provide digital linear encoder designs for one dimensional transverse motion as exemplified by Andermo in U.S. Pat. No. 4,420,754.
However, the prior methods are inapplicable to the present case which is new and different in four important respects. First, the position encoder needs to be truly two dimensional, reading out either or both the X and Y position simultaneously with orthogonal independence, i.e., translation in X not affecting the Y readout and conversely. Second, the position measurement system must not require the use of any special wiring, interconnections or electrically floating electrodes associated with the platen itself. Third, the outer information needs to be independent of electrode spacing and any associated "roll", "pitch" or "yaw" of the sensor. And finally the sensor needs to be of sufficiently large area that it averages over hundreds, if not thousands, of substrate posts. (This last requirement relieves the need for unreasonable demands being placed on the post-to-post uniformity of the platen for high positioning accuracy.) Additional desiderata are low sensor fabrication cost, fast response, independent height readout (indicating instantaneous air bearing thickness) and simple associated readout electronics.